In the manufacture of homopolystyrene, and elastomer-modified versions thereof of the type containing graft polymerized diene rubber particles dispersed in a homopolystyrene matrix, especially by continuous mass polymerization methods, it is necessary after polymerization to separate from the polymer unreacted styrene monomer. To accomplish such a separation, it is heretofore been common to boil off the styrene monomer from the polymer under conditions of elevated temperature and (preferably) sub-atmospheric pressures, this process commonly being termed devolatilization.
One class of apparatus used for styrene devolatilization involves the combination of a preheater, which is typically a shell and tube heat exchanger, and a flash tank connected therewith. Thus, a mixture of homopolystyrene and styrene monomer (with or without grafted diene rubber distributed therein) in the form of a hot melt is continuously passed through the tubes of the preheater and heated to a temperature above the boiling point of styrene monomer but below the decomposition (or vaporization) point of the styrene polymer. Thereafter, this heated mixture is passed directly into an upper portion of the flash tank. The interior of the flash tank is maintained at elevated temperatures and (preferably) reduced pressures. The melt mixture passed into the flash tank is caused to drop through a region of the flash tank in the form of strands or globules. As a result, most of the unreacted monomer is removed continuously as a vapor from a vent in the tank which is usually located above the melt input port in the tank. Such an apparatus is termed a falling strand devolatilizer.
Falling strand devolatilization, however, has heretofore had associated with it a serious drawback in that such devolatilization has been observed to produce in the devolatilized product significant quantities of oligomers (dimers and trimers of styrene). The concentration of such oligomers in such product polymer can be such as to alter polymer product properties adversely which results in the production of an off-grade material. This oligomer production problem during falling strand devolatilization has tended to limit utilization of falling strand devolatilization when manufacturing homopolystyrene products particularly using mass polymerization technology.
All styrene polymer have some small amount of oligomers associated with them. The method of utilizing a preheater in a process of vaporizing the monomer inherently generates more oligomers than does certain other types of devolatilization methods such as wiped film devolatilization. Oligomer production in falling strand devoltilization tends to result because preheater tube surfaces through which heat passes to heat up the polymer melt passing therethough are not wiped clean which tends to promote the production of oligomers in the relatively larger layer of melt which tends to adhere near the tube inner wall surfaces.
It has now been discovered that by the use of an unconventional type of shell and tube preheater in a falling strand devolatilization technique, one can reduce the relative quantity (to total polymer) of oligomers made to a level such that the quantity of oligomers in the devolatilized product is not greater than the amount of oligomers in polymer product made by wiped type devolatilizaton. Specifically, the present discovery relies on the fact that by using short length tubes having an appropriate internal diameter in combination with associated carefully selected preconditions for the flash chamber into which the heated stream from the tubes of the preheater are deposited results in the control and minimization if desired of low molecular weight polymer and oligomers.
It is thus an object of the present invention to provide apparatus and processes enabling one to carry out falling strand devolatilization on a homopolystyrene/styrene monomer composition without concurrently making excessive offgrade polymer or oligomers.
Another object of the present invention is to provide an apparatus in which one may carry out falling strand devolatilization of homopolystyrene polymer/styrene monomer mixture to remove styrene monomer from polymer while simultaneously controlling at different levels the amount of polymer in the starting composition which is inherently converted to low molecular weight polymeric material during devolatilizaion, as well as processes for utilizing such apparatus and the accomplishment of such objective. Other further objects of this invention will occur to those skilled in the art from a reading of the present specification and drawings taken with the claims.